Conventionally, a film screen system which uses an intensifying screen in combination with radiographic film has been used for picking up radiation images. According to this method, a radiation ray that has transmitted through an subject to be photographed and thereby contains information about the subject's internal condition is converted by an intensifying screen to visible light in proportion to the radiation intensity, and then the radiographic film is exposed to the visible light, thereby a radiation image is formed on the film. However, such a film system is laborious because of the necessity of a development process; accordingly it takes time for a doctor to finally receive a radiographed radiation image.
Due to recent technological progress, in the medical industry, there is an increasing request for receiving radiation image information as a direct electrical signal without using radiographic film. That is, a radiation image pickup apparatus has been introduced which includes a semiconductor sensor that can directly convert a radiation ray into an electrical signal in proportion to the radiation intensity; or includes an indirect-type semiconductor sensor that converts a radiation ray into visible light by fluorescent substance in proportion to the radiation intensity and then converts the visible light into an electrical signal by a photoelectric conversion element; and further includes a flat panel detector (hereafter referred to as FPD) which reads out the converted electrical signal by a circuit that uses an amorphous silicon thin-film transistor (a-TFT).
The FPD will be described with reference to FIG. 8 and FIG. 9.
Generally, an indirect-type FPD is configured such that a fluorescent plate which converts a radiation ray into visible light, a photoelectric conversion element which converts visible light into an electrical signal, a TFT switch circuit, and wires which connect those elements are formed on a glass substrate. FIG. 8 shows the FPD 80 circuit and peripheral circuitry. In FIG. 8, number 3 represents a photoelectric conversion element that uses a PIN-type photodiode, and number 2 represents an amorphous silicon thin-film transistor wherein its drain electrode is connected to a readout line 22a, 22b . . . 22c, its source electrode is connected to the cathode of the photoelectric conversion element 3, and its gate electrode is connected to a gate line 21a, 21b and 21c. The anode of the photoelectric conversion element 3 is connected to a bias line 15 which is connected to a bias circuit 81, and negative bias voltage is applied. Gate lines 21a, 21b and 21c are connected to the output terminals G1, G2, . . . GN of the gate driver circuit 82, respectively, that functions as a scanning section; and readout lines 22a, 22b . . . 22c are connected to the output terminals S1, S2 . . . SM of the readout circuit 83, respectively. In this FPD 80, a combination of one photoelectric conversion element 3 and one thin-film transistor 2 constitutes one pixel, and altogether pixels of N lines multiplied by M columns (N and M are positive integers and M, N≧2) are included.
The output terminals G1, G2, . . . GN of the gate driver circuit 82 are connected to the gate lines 21a, 21b, . . . 21c, respectively, and the gate driver circuit 82 outputs positive voltage in sequence thereby scanning the gate lines 21a, 21b, . . . 21c. The output terminals S1, S2, . . . SM of the readout circuit 83 are connected to the readout lines 22a, 22b, . . . 22c, respectively, and the readout circuit 83 outputs positive voltage. Furthermore, each of the output terminals S1, S2, . . . SM of the readout circuit 83 includes a charge-voltage conversion circuit which converts an amount of electric charges flowing out to the readout line 22a, 22b, . . . 22c into voltage.
The fluorescent plate (not shown) is configured such that it covers the above FPD 80 and it allows visible light generated on the fluorescent plate by an exposure to radiation to enter the photoelectric conversion element 3.
Next, operations of the above FPD will be described.
FIG. 9 is a timing chart that shows operations of the FPD 80. In FIG. 9, waveforms denoted by G1, G2 . . . GN show voltages of the output terminals G1, G2 . . . GN of the gate driver circuit 82, respectively. When the voltage of the gate lines 21a, 21b, . . . 21c becomes high, all of the thin-film transistors 2 connected to the gate line are turned on. At this point in time, because the output terminals S1, S2, . . . SM of the readout circuit 83 output positive voltage to the readout lines 22a, 22b, . . . 22c, the photoelectric conversion elements 3 connected to the activated thin-film transistors 2 are reversely biased, thereby charging the capacity of the photoelectric conversion elements 3. At this point in time, charging current that flows into the photoelectric conversion element 3, which is an electric charge that flows into the readout line 22a, 22b, . . . 22c from the output terminal S1, S2, . . . SM of the readout circuit 83, is converted from an electric charge into voltage by the readout circuit 83 and read out as a voltage. When the voltage of the gate lines 21a, 21b, . . . 21c becomes low, all of the thin-film transistors 2 connected to the gate lines are turned off, the charging voltage of the photoelectric conversion elements 3 connected to the thin-film transistors 2 is maintained.
An operation indicated as “initialization scanning” in FIG. 9 is a scanning operation for initializing the FPD by charging all of the photoelectric conversion elements 3 in preparation of picking up radiation images. A waveform indicated as “X-ray exposure” in FIG. 9 shows an exposure to a radiation ray, the period during which the voltage is high is the period when radiation exposure is being conducted. As shown in FIG. 9, radiation exposure is conducted after the initialization scanning of the FPD 80 has been finished. When the radiation exposure process starts, a fluorescent plate that is exposed to radiation starts to emit fluorescent light, and an electron-hole pair is generated in the photoelectric conversion element 3 that has received the fluorescent light, thereby discharges electric charges stored therein. Accordingly, electric charges that have been stored in the photoelectric conversion element 3 decrease by an amount equivalent to the number of electron-hole pairs generated.
After the radiation exposure process, readout scanning is conducted as shown in FIG. 9. The voltage that has been converted from electric charge and read out from the readout circuit 83 during the readout scanning operation is equivalent to the amount of electric charges that have been discharged from the photoelectric conversion element 3 during the radiation exposure operation. Thus, a radiation image entered onto a fluorescent plate is two-dimensionally read out as a voltage.
“ti” in FIG. 9 represents integration time; electron-hole pairs caused by visible light generated by the fluorescent plate discharge electric charges stored in the photoelectric conversion element 3 by the amount equivalent to that of generated electron-hole pairs, and the electron-hole pairs generated during this period are substantially integrated by the photoelectric conversion element 3. It is preferable that the integration time should include the radiation exposure period and the fluorescent plate's light-emitting period.
With regard to the operation state of the FPD, two states can be considered: the operating state when voltage is being applied to the FPD and the standby state when voltage is not applied. In this case, in the standby state, the electric charges are trapped at the photoelectric conversion element's trap level by thermoelectrons and become dark current components. For this reason, when using the FPD, at the time the status is switched to the operating state, the photoelectric conversion element is initialized (reset operation) to discharge electric charges stored therein. This is done by applying reverse bias to the photoelectric conversion element for a predetermined time. At this point in time, the amount of current that flows as the result of the application of reverse bias gradually decreases as time goes by. This current results in noise when an image is radiographed, and sometimes it is necessary to apply reverse bias for several tens of seconds until the amount of current decreases to the degree that does not cause problem with picking up images.
In a conventional image pickup apparatus, because the power is supplied from outside by a cable, in order to apply reverse bias full-time to the photoelectric conversion element, the FPD has to be scanned all the time, or an electric charge readout circuit connected to the FPD has to be operated all the time while all of the switching thin-film transistors connected to the photoelectric conversion elements are turned on.
However, with this kind of operation method, there is a problem that the amount of current consumed tends to become great. That is, each readout line of the readout circuit uses an operational amplifier circuit. For example, if each operational amplifier consumes 3 mW of power, since the FPD generally has 2000 readout lines, the total amount of power consumption amounts to 6 W, resulting in 48 Wh a day when those amplifiers are operated 8 hours a day.
If the equivalent amount of power is to be provided by a battery, and the power density of a general lithium-ion secondary battery is 150 Wh/kg, a battery of at least 300 g is necessary to simply maintain the initializing state. If the battery capacity is increased for the cases when the operating time is extended or to take into consideration that the battery's ability to store energy may deteriorate over time, the required weight of the battery increases. When a portable image pickup apparatus that uses an FPD is assumed, the weight of the battery becomes a key factor. If some of the battery capacity has to be allocated to the power consumed during the standby state, feasibility of a portable apparatus that uses a battery power supply significantly decreases. Furthermore, other than the weight of the battery, a problem arises in that heat generated by the readout circuit increases the FPD's dark current or fluctuates the offset.
As an example of a conventional technology that copes with these problems, there is a disclosed technology wherein two power supply circuits are separately provided: one as a power supply for applying bias to an FPD and the other as a power supply for a readout circuit; and a power supply for bias application is first operated according to the generation of the radiographing preparation request signal to apply bias to the photoelectric conversion element thereby initializing the FPD; and a power supply for the readout circuit is operated after the X-ray exposure process so as to reduce power consumption (see patent document 1). [Patent document 1] Japanese Laid-Open Patent Publication No. 2002-165142.
However, according to the technology disclosed in patent document 1, only a period of time from the generation of the radiographing preparation request signal to the start of X-ray exposure can be used for initializing an FPD. Usually, this time period is only several seconds which is too short because several tens of seconds are usually required to fully initialize an FPD. For this reason, dark current components stored in the photoelectric conversion element, specifically electric charges trapped at the trap level, are not sufficiently discharged, which makes it difficult to sufficiently reduce noise components of the picked up image.